1. MOTIVATION AND OUTLINE

Spiral galaxies consist of three spatially and dynamically dinstinct
components, namely bulge, disk, and halo. Of these, the central bulge
consists primarily of old stars and has a low gas fraction. The
disk and halo consist of stars plus gas, dust, and in many cases ``dark
matter''. Here I want to concentrate on the interstellar medium (ISM)
in the disks and halos of galaxies, with emphasis on late-type spirals.
Massive dark halos will not be described (see e.g.
Ashman 1992;
Holt and Bennett
1995).
In the current context ``late-type'' stands
for disk-dominated spirals with rather inconspicuous bulges (Hubble
types Sb - Sdm). The ISM of elliptical galaxies and early-type
spirals (S0 - Sab) will also not be presented here (cf.
Walsh et al. 1989 and
Fabbiano 1989).

Studies of the ISM contribute significantly to our understanding of
the structure, kinematics, and evolution of galaxies. The level of
star formation (SF), its spatial distribution, and its temporal
changes depend strongly on the properties of the interstellar gas
from which stars can form. Stars feed energy and metal-enriched
matter back into the ambient medium via stellar winds and supernovae
(SNe), thereby distributing processed material throughout the ISM.
Hence, through this gas-star-gas cycle, the ISM becomes a mediator
of SF.

In Section 2, the different phases of the ISM in
galaxies will be introduced, and Section 3 presents
observations of the ISM in the Milky Way. Section 4
will show that the Galactic ISM is not confined to the thin disk,
but extends to high distances (z) above the plane, forming a
gaseous halo. However, being located within the disk of the Milky
Way, we lack a grand overview. Therefore, many global parameters of
interstellar matter can be better and more easily determined by
looking at external systems. For spatial investigations of galaxy
disks a face-on viewing geometry is favorable, while for investigations
of the vertical structure of galaxies edge-on systems are best-suited.
In the following paragraphs, I will give a summary of our observational
knowledge about the ISM in both the disks
(Section 5)
and the halos (Section 6) of external spiral
galaxies and provide examples of key observations.
Section 7
summarizes the observational evidence for the complex nature of the
halo ISM in external galaxies, while
Section 8 provides
a brief account of how theoretical models describe the properties
of interstellar matter, including disk-halo interactions in both
starburst and ``normal'' spiral galaxies. The goal of this article
is to show that it takes a certain minimal amount of energy input -
coming from SF processes - in the disks of spiral galaxies to initiate
outflows creating gaseous halos. Outflow phenomena being the primary
cause for the existence of gaseous halos, the halo ISM of late-type
galaxies can then be understood as a natural extension of the ISM in
the galaxy disks.